As the control system, for example, if a water level control system that controls an in-tank water level configured by using a PLC (programmable logic controller) is taken as an example, the load as the control target is an electromagnetic valve that controls a flow rate of water. There are various types of electromagnetic valves. However, most of the types control opening and closing of the valve with an electric current of 4 to 20 milliamperes. In general, a specification range of the resistance of a current receiving section for 4 to 20 milliamperes provided in the electromagnetic valves is equal to or smaller than 600 ohms.
The analog current output circuit that supplies the analog current to the load has a configuration in which an output transistor and the load are arranged in series in this order between a power supply and the ground and a passing current of the output transistor is controlled such that an analog current coinciding with an output current command value flows to the load.
The resistance of the electromagnetic value serving as the load connected to the analog current output circuit is different according to a type of the electromagnetic valve within the specification range (e.g., 0 ohm to 600 ohms). Therefore, a voltage of power supplied to the analog current output circuit needs to be set to a sufficiently high voltage.
Therefore, when the resistance of the load is small and an analog output current is large, a power loss in the output transistor increases, and thus, internal heat generation of the output transistor due to the increase in this power loss has conventionally posed a serious problem.
To cope with the problem, for example, Patent Document 1 proposes a method of using a low-loss power supply of a switching type, continuously changing a power voltage of the power supply, and reducing a power loss in an output transistor. Specifically, in a technology disclosed in Patent Document 1, a step-down circuit of the switching type is used in the power supply, a potential difference between a voltage supplied to a collector of the output transistor from the step-down circuit and an output terminal, to which an emitter terminal is connected, is detected by an OP amplifier, and the potential difference is given to the step-down circuit of the switching type as a Ref voltage. Consequently, a collector-to-emitter voltage Vice of the output transistor is always kept at a fixed value of about +1 V to reduce power consumption of the output transistor.
Patent Document 1: Japanese Patent Application Laid-open No. 2000-252754